This invention deals generally with filament reinforced thermoplastic pre-impregnated tape, and more specifically with a multiple directional and multiple layer filament reinforced thermoplastic tape for use in pultrusion processes.
Mixtures of various filaments and plastic resins are used to produce composites that have unique properties compared to the traditional engineering materials like metals and non-reinforced plastics. The filaments in the resin matrix increase the strength of the composites so much that they far exceed even the strongest metals, although composites are much lighter than their metal counterparts.
Thermoplastic resins are often used to make these composites. Thermoplastic resins are solid at room temperature, and they soften and ultimately melt at elevated temperatures turning into a very viscous liquid. Reinforced thermoplastic composites are typically produced by intermingling finely ground resin particles with bundles of filaments and elevating the temperature above the resin's melting point, but below the filament melting point. This causes the resin to wet the filaments and to encapsulate them in the resin. Conventionally, the filament bundles must be opened up before wetting can occur, but the filaments remain aligned in the direction of the tape.
This form of filament reinforced pre-impregnated resin composite is called a prepreg. It is usually made into a simple sheet like form that is the basic component subsequently used to make more complex finished parts. The prepreg is very strong in the filament direction, but is relatively weak in the transverse direction.
One of the products produced from this process is called prepreg tape which has a width dimension significantly smaller than its length. Reinforced thermoplastic prepreg tape can be used in a pultrusion process in which several layers of tape first pass through a heater to remelt the resin. The soft viscous column of resin and filaments then passes through a die, which forms the final desired shape, fusing the resin from all the layers together, after which the composite hardens into its final shape. Various shapes such as solid, hollow, round, square, flat, or irregular can be produced. In most cases the finished part requires its highest strength in the length direction of the part, which is the direction the filaments would be naturally oriented.
However, there are some specialized applications where the highest strength is required at 90 degrees from the length direction of the part. In these cases a special layup of prepreg is required to get a substantial amount of the filaments oriented 90 degree from the length direction of the pultrusion. For example, a long rectangular cross section part such as a 0.5″ thick×4″ wide×36″ long slab which is loaded so the stress lines run laterally requires that a substantial amount of filaments be oriented in the width direction, perpendicular to the thickness and length directions.
Presently, making a suitable tape for such a structure with the reinforcing filaments running across the width requires first slitting conventional unidirectional reinforced prepreg tape into strips of the desired 4 inch width of the tape to make a base strip that is spooled. Subsequently cross pieces of unidirectional prepreg tape are cut to a length matching the 4 inch width of the base strip and placed on top of the base strip with the reinforcing filaments of the cross pieces oriented at 90 degrees to the reinforcing filaments of the base strip. The short cross pieces are then attached to the base strip, typically by spot welding the short pieces to the base strip, to form a two layer tape.
Multiples of these two layer tapes are stacked and then used as the feed material for the pultruding process. In a typical example, there could be nearly 25 two layer tapes used to make the finished part. The 25 two layer layers would be pulled like a stack of pages in a book through the die.
This process is labor intensive and less than ideal. Frequently the spot welds do not hold when the two layer coils are being prepared. The pieces then must be manually reattached. In other cases the pieces may fall off, after previously being attached, and they have to be manually retrieved and reattached before entering the pultruder preheater. If a piece is absent in the composite, the finished product is defective.
In such tapes the quantity of filaments placed in the transverse direction is limited to be no greater than that placed in the longitudinal direction. Therefore, the final product is only as strong in the transverse direction as it is in the longitudinal direction. The resulting composite is inefficient because the 50% of the filament oriented in the lengthwise direction is not used to carry the bending load across the width direction. Ideally, for bending loads nearly all the filaments should be in the transverse direction, not just half of them.
Another problem is that the alternating layers of 0 and 90 degree filaments must be arranged to achieve a balanced design. However, using the two layer tapes, an operator error is easily possible. If two adjacent tapes are oriented so either their 90 degree or 0 degree filament layers face each other then the finished product will have two 90 degree layers and two 0 degree layers next to each other rather than the desired alternating 0 and 90 degree layers. This results in a product flaw.
Furthermore, in order for the stack of tapes to slide properly on the preheater or pultruder die surfaces, an extra layer of 0 degree prepreg must be on the top or bottom surface of the stack of tapes to cover the 90 degree layer that should not be an outside layer. This extra 0 degree layer protects the otherwise exposed 90 degree layer from rolling up in the pultruding process and jamming the pultruder. However, because both outside layers must be 0 degree layers, the first load bearing cross filament 90 degree layer is at least one full layer thickness away from the outer surface where it would be most effective in carrying the bending load.
Components made using a 0 and 90 degree prepreg orientation can also exhibit shear failures when subjected to bending. This failure mode is most prevalent in products made from prepreg that has high filament loading and low resin content. High filament loading is typically used in products requiring high strength. These failures typically occur at the interface between the 0 and 90 degree layers at the center of the bending plane; but usually never within a composite layer itself. This shear failure condition can limit the ultimate strength of the part.
It would be very beneficial to have a prepreg tape that overcomes this problem of shear failure due to delamination of the layers.